1. Field of the Invention
The present invention relates to a multi-color X-ray generator which successively switches and generates two or more types of monochromatic hard X-rays at short time intervals
2. Description of the Related Art
X-rays are electromagnetic waves having a wavelength of about 0.1 to 100 A (10−11 to 10−8 m), among the rays, an X-ray having a short wavelength (10 to 100 keV, λ=1 to 0.1 A) is referred to as a hard X-ray, and an X-ray having a long wavelength (0.1 to 10 keV, λ=100 to 1 A) is referred to as a soft X-ray. Moreover, an X-ray emitted at a time when an electron beam or the like is struck on a substance and having a wavelength inherent in a constituting element of the substance is referred to as a particular X-ray.
As apparatuses in which the X-rays are used, an X-ray transmission apparatus, an X-ray CT apparatus, an X-ray diffraction apparatus, an X-ray spectral apparatus and the like are utilized in broad fields such as a medical treatment, bioscience and material science. For example, to cure cardiac infarction, coronary angiography (IVCAG) in which an X-ray of about 50 keV is used is generally performed. Moreover, the X-ray CT apparatus is an apparatus in which an object to be measured is irradiated with X-rays from different directions to measure absorption of the rays, and an image is reconstructed by a computer to obtain a two-dimensional sectional image of the object.
As generation sources of the X-rays, an X-ray tube and synchrotron radiation light are known.
The X-ray tube is a device in which a thermion obtained by heating a filament in vacuum is accelerated at a high voltage, and is allowed to collide with a metal anode (a target), thereby generating the X-ray. Examples of the X-ray to be generated from the X-ray tube include a continuous X-ray obtained by braking radiation of an electron, and a particular X-ray which is a bright line spectrum. The continuous X-ray is used as a ray source for an application in which any X-ray having a specific wavelength is not required, for example, a transmission process for a medical treatment or industry. The particular X-ray is used for an application in which the X-ray having the specific wavelength is required, for example, X-ray diffraction, fluorescent X-ray spectroscopy or the like.
On the other hand, the synchrotron radiation light (SR light) is an X-ray generated during an orbit change in a case where an orbit of the electron beam accelerated at a speed close to a light speed is changed by a strong magnet in an annular accelerator (a synchrotron). The SR light is an X-ray source (e.g., an X-ray intensity (a photon number): about 1014 photons/s, a pulse width: about 100 ps) which is incommensurably (103 times or more) intense as compared with the X-ray tube, and the ray is used for a field in which a high X-ray intensity is required.
However, a synchrotron radiation light facility in which a synchrotron is used is a large-sized facility in which the synchrotron has a large diameter of about 50 m or more and an orbit length reaches 100 m or more, and there is therefore a problem that the facility even for a research or the medical treatment cannot easily be introduced. To solve the problem, a small-sized X-ray generation device is proposed in which a small-sized linear accelerator is used (e.g., Non-Patent Document 1).
On the other hand, in a conventional X-ray CT apparatus, a monochromatic meter including two crystal plates is used as means for obtaining a monochromatic hard X-ray from the radiation light. Since the monochromatic X-ray CT apparatus has a low measurement precision of an electron density, a mixed two-color X-ray CT apparatus is proposed in which two types of X-rays having different mixture ratios of a dominant wave and a higher harmonic wave are used (e.g., Non-Patent Document 2).
In addition, as means for generating two types of X-rays, Patent Documents 1, 2 have already been disclosed.
In “Small-Sized X-Ray Generation Device” of Non-Patent Document 1, as shown in FIG. 1, an electron beam 52 accelerated by a small-sized accelerator 51 (an X-band acceleration tube) is allowed to collide with laser 53 to generate an X-ray 54. The multi-bunch electron beam 52 generated by an RF electron gun 55 (a thermal RF gun) is accelerated by the X-band acceleration tube 51, and collides with the pulse laser light 53. The hard X-ray 54 having a time width of 10 ns is generated by Compton scattering.
This device is miniaturized by using, as an RF, an X-band (11.424 GHz) corresponding to a frequency four times as high as that of an S-band (2.856 GHz) for general use in a linear accelerator, and it is predicted that the hard X-ray having, for example, an X-ray intensity (the photon number) of about 1×109 photons/s and a pulse width of about 10 ps is generated.
As shown in FIG. 2, “Mixed Two-Color X-Ray CT Apparatus” of Non-Patent Document 2 includes a rotary filter 61, a monochromatic meter 62, a collimator 63, a transmission type ion chamber 64, a scattering member 65, a sliding rotary table 66, an NaI detector 67 and a plastic scintillation counter 68. A dominant wave X-ray of 40 keV and a double higher harmonic wave X-ray of 80 keV are extracted from synchrotron radiation light 69a by the monochromatic meter 62, a mixture ratio of the 40 keV X-ray and the 80 keV X-ray is regulated by the rotary filter 61, scattered X-ray spectrum from the scattering member 65 is observed by the NaI detector 67 to measure the mixture ratio, a size of a mixed two-color X-ray 69b is adjusted by the collimator 63, the ray is transmitted through the transmission type ion chamber 64 and a subject 60, and an intensity of the ray is measured by the plastic scintillation counter 68.
According to this apparatus, the measurement precision of the electron density is improved, and the apparatus is successful in preparation of an image indicating the electron density and an effective atomic number.
As shown in FIG. 3, “X-Ray Generation Device” of Patent Document 1 has a microtron 73 which defines a plurality of electron circulation orbits partially shared and which is provided with an accelerator 72 to increase and reduce energy of an electron beam 71 in the shared part of the orbits, an electron beam source 74 which strikes the electron beam 71 on the electron circulation orbit of the microtron, and a laser light source 76 which emits a laser light 75 so that the laser light collides with an electron that flies at the shared part of the orbit of the microtron.
As shown in FIGS. 4A to 4C, “X-ray Generation Device” of Patent Document 2 has an electron beam source 82 which emits a pulse-like electron beam 81, and laser optical systems 85A, 85B which emit first and second laser lights 83A, 83B in a pulse-like manner in synchronization with the emission of the pulse-like electron beam so that the laser lights collide with the pulse-like electron beam emitted from the electron beam source.
[Non-Patent Document 1]
“Development of Small-Sized Hard X-Ray Source using X-band Liniac”, 2002, authored by Katsuhiro DOHASHI, et al.
[Non-Patent Document 2]
“Development of Mixed Two-Color X-Ray CT System” authored by Makoto SASAKI, et al., Medical Physics Vol. 23 Supplement No. 2 Apr. 2003
[Patent Document 1]
Japanese Patent Application Laid-Open No. 2002-280200 titled “X-Ray Generation Device and Generation Method”
[Patent Document 2]
Japanese Patent Application Laid-Open No. 11-264899 titled “Electron/Laser Collision Type X-Ray Generation Device and X-ray Generation Method”
In angiography by a difference process and two-color X-ray CT, a switch speed of X-ray energy is important. For example, in dynamic angiography, the energy needs to be switched to obtain an image in a short time to such an extent that it can be judged that a blood vessel does not move. In the two-color X-ray CT, if much time is required for the switching of the energy, there is a problem that a state of a subject changes and a quality of the reconstructed image drops.
To obtain the monochromatic hard X-rays from the radiation light by use of the monochromatic meter, since the monochromatic meter includes two crystal plates as described in Non-Patent Document 2, two types of monochromatic meters need to be used to obtain two types of monochromatic hard X-rays (the two-color X-ray). However, in the monochromatic meter, since a crystal angle needs to be precisely adjusted, it is very difficult to switch the monochromatic meter in the short time.
Moreover, even in another X-ray source in which the particular X-ray is used as a monochromatic X-ray, the target needs to be switched physically, and it is also difficult to switch the target at the high speed.
Furthermore, in a case where the mixed two-color X-ray obtained by mixing the dominant wave X-ray and the double higher harmonic wave X-ray are mixed is extracted from the synchrotron radiation light as in Non-Patent Document 2, the wavelength of the X-ray is limited to that of the higher harmonic wave, and there is also a problem that the dominant wave cannot be separated from the higher harmonic wave.
In addition, in “X-ray Generation Device” of Patent Document 1, the wavelength of the laser light cannot be switched in the short time.
Moreover, “X-ray Generation Device” of Patent Document 2 has problems that the device has a small collision probability between the pulse-like electron beam and the first and second laser lights and a low X-ray generation output.
To solve the above problems, the present invention has been developed. That is, an object of the present invention is provide a multi-color X-ray generator capable of successively switching and generating a plurality of (two, three or more types) monochromatic hard X-rays at short time intervals to such an extent that it can be judged that a blood vessel does not move and capable of generating an intense X-ray applicable to angiography or the like.